I agree about Hivemail's spelling issues... it always make me cringe when I see stuff like that. But price is another matter. I've been looking at other email server options for a while. Hivemail is a close comparison to most and is far cheaper.
However, not to be negative about your quest, I can't help but wonder
why you're going down a road that someone else is already on. Hivemail has just begun vBulletin integration and will probably improve it with every version. Hivemail is already a few steps ahead of you and it'll take a lot to catch up. By then, Hivemail may cover all the concerns that you have about their software.
Maybe go into a different direction with vBulletin... possibly a truely integrated chat room, classifieds section, or auctions? Somewhere someone doesn't already have hold on.
Hivemail had an 80% off sale last week that seemed more desperate then planned. Maybe there's not as large of a target audience? Would there be room for two?
Anyway, I've installed more of your hacks then I can count so I certainly won't question your abilities.

Like I said, I don't want to be negative, but I'm just curious.
Good luck! I'll definitely be watching.